It is known that the link's power budget of a radio communication device is greatly enhanced when beamforming is used. As shown in FIG. 1, beamforming involves the use of multiple antennas (as in a phased array). In a transmitter the signal is first distributed over the antennas and then delayed (or phase shifted), where the delay defines the direction of signal transmission, while in the receiver the signal in each antenna path is first delayed, where the delay depends on the direction of reception, and then combined. It is the task of the beamformer to create these delays and add them to the signals of the respective antenna paths.
In the case of narrowband radio communication the delays can be approximated by phase shifts. To realize these phase shifts, circuits called phase shifters (or beamformers) are implemented, operating in one of the major domains of a radio device as shown in FIG. 2. That is, in the radio frequency (RE) domain, in the local oscillator (LO) domain, in the intermediate frequency (IF) domain (not shown in the figure) or in the baseband (BB) domain. In case of a direct-conversion system, the IF domain does not exist; phase shifters are then provided in at least two of the other domains. In any case, the antenna-referred phase shift has to have a range of 360 degrees, to be able to realize any direction of transmission.
Beamforming (BF) applied directly at the radio frequency (RE) (FIG. 2a) offers the benefit that the duplication of the different signal operations in a transceiver is kept to a minimum. However, in semiconductor technologies, e.g. in digital CMOS, beamforming at radio frequencies yields high losses which, in addition, depend on the desired phase shift. Moreover, this approach is sensitive to small layout parasitics. These disadvantages render the current RF beamforming techniques not suitable for low noise and ultra-low power radios.
In LO phase shifting (FIG. 2b) the phase shift is applied to the LO signal and not in the signal path. In a receiver the high-frequency signal is down-converted with a LO signal that is phase shifted with respect to the LO signals for the other antenna paths. Therefore, multiplication of the LO paths may be desirable, as every mixer in each antenna path needs to be steered by a phase shifted version of the LO signal. After down-conversion, the signals of the different antenna paths are in phase and they can be combined, yielding a signal quality improvement. The implementation in the transmitter may include a split of the signals over the different antenna paths before up-conversion. Then, in each antenna path an up-conversion is performed with a LO signal that is phase shifted with respect to the LO signals in the other antenna paths. Compared to beamforming at radio frequencies (RF), beamforming in the LO path implies a duplication of the down-conversion or up-conversion mixers and routing of the LO signal to the different phase shifters. Just as with RF beamforming, high-frequency power hungry phase shifters are needed, but the noise and gain requirements are alleviated. Again, due to the elevated power consumption, this LO beamforming technique is not suitable for low noise and ultra-low power radio application.
In baseband beamforming the beamforming in the baseband (BB) path can be implemented in analog or digital domain. In analog baseband beamforming (FIG. 2c) in systems featuring in-phase and quadrature signalling, the phase-shift adjustment is performed by implementing the operation of matrix rotation of the constellation on a complex plane. The rotation of constellation is equivalent to phase shift when the signal is translated to RF domain (up/down-converted). This operation can be implemented with a set of variable-gain amplifiers, where the rotation of the complex constellation plane is controlled by varying the gain factors of the amplifiers. The beamforming in the digital baseband path (not shown in the figure) can be implemented following the same principle. However, this may include a duplication of the complete analog functionality of a radio (filters, variable-gain amplifiers, ADCs) over all antenna paths, as for every antenna path a dedicated digital path is necessary together with a dedicated digital/analog converter. This in turn leads to excessive power consumption.
For wireless communication at high data rates the 57-66 GHz frequency band is allocated. Transceivers for such communication can advantageously be implemented using highly downscaled CMOS.
In comparison to the RF and LO beamforming implementations, the BB beamforming is the most suitable for CMOS implementations, as it offers improved flexibility, reduced power consumption and area. However, the BB beamforming scenario is not suitable for simple transmission schemes, for example binary phase shift keying (BPSK) and on-off keying (OOK) schemes, where only in-phase signals are used, because it is specifically suited for operation with a quadrature signal, i.e. a signal with in-band (I) and quadrature (Q) components. Moreover, the introduction of variable-gain amplifiers and signal combiners into the baseband path inevitably reduces the signal quality.
To combat the issues of implementing a full-range phase shifter, a combination of a quadrant selection (coarse phase tuning) using phase-shift local oscillator and fine phase-shifting using RF phase shifters technique has been proposed by Chu et al. (“CMOS phase-shifting circuits for wireless beamforming transmitters”, Analog Integrated Circuits and Signal Processing, 2008, Vol. 54 (1), pp. 45-54). However, the proposed hybrid beamforming scheme still suffers from high power consumption, high complexity of the circuit, which in turn brings high influence on the quality of the processed signal, and therefore high signal distortion.
It is apparent from the above, that conventional beamforming techniques suffer from high complexity, high power consumption and usually high signal quality degradation. This is mostly because a complex circuit is inserted in the path of the signal. Often, such circuit is implemented so that it introduces losses to the path, inevitably increasing noise and requiring additional amplification. Even if the phase shifting circuitry is not placed in the signal path, but in the path of the local oscillator, the requirement of full phase shifting range brings higher power consumption, area consumption and complexity.
Hence, there is a need for solutions for performing beamforming wherein these problems are overcome.